I put a storage chest at my docks, which are outside my walls, and I built a low stone wall around it with a hatch on top to access the chest. That's been safe out there so far.

I assume chests themselves are not safe if not blocked from view? I saw a greydwarf go crazy on one of those stone tower ruins and destroy a chest that wasn't even one I built.

(The hatch door is from a mod).